Shoe Goo. Works very well and widely available.
I had major sole separation on a pair of motocross boots. I was amazed at how well Shoe Goo worked to re-bond the soles.
07 Tacoma V6 - M1 EP 5W-30, Fram Ultra, 10K OCIs, 223K
18 Civic HB 1.5T 6MT (Fiancee's) - Castrol Edge 0W-20, OEM filter
07 Yamaha R1 - Rotella T6 5W-40, Bosch 3300
07 Yamaha YZ-250, Klotz, Rotella